Fair Value - Carrying Amounts and Estimated Fair Values of Financial Instruments, Not Carried at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	$ 286,761	$ 290,432	$ 252,367	$ 255,519
Investment securities held to maturity	512,207	684,063	688,951
FHLB/Federal Reserve Bank stock	136,206	119,686	117,354
Commercial	2,949,277	2,717,269	2,049,054
Commercial real estate	4,481,554	4,354,552	3,370,211
Residential real estate	2,166,203	2,167,053	2,119,120
Consumer credit	1,695,625	1,879,247	1,859,739
Accrued interest receivable	78,493	87,102	75,342
Noninterest-bearing demand deposits	3,588,370	3,680,807	3,034,696
Time deposits	1,892,135	1,634,436	1,451,989
Federal funds purchased and interbank borrowings	450,031	335,033	317,021
Securities sold under agreements to repurchase	319,831	384,810	$ 285,409
Other borrowings	1,554,515	1,609,579
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	286,761	290,432
Accrued interest receivable	34	16
Noninterest-bearing demand deposits	3,588,370	3,680,807
Checking, NOW, savings, and money market interest-bearing deposits	7,117,695	7,290,521
Federal funds purchased and interbank borrowings	450,031	335,033
Securities sold under agreements to repurchase	319,831	359,810
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued interest receivable	21,556	24,001
Time deposits	1,889,488	1,620,685
Securities sold under agreements to repurchase		25,133
Other borrowings	248,749	250,443
Accrued interest payable	6,582	7,029
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial	2,871,624	2,707,385
Commercial real estate	4,374,201	4,347,949
Residential real estate	2,117,866	2,210,951
Consumer credit	1,646,883	1,998,194
Accrued interest receivable	56,903	63,085
FHLB advances	1,540,755	1,607,189
Standby letters of credit	445	351
Commitments to extend credit	3,209	2,449
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, due from banks, money market, and other interest-earning investments	286,761	290,432
FHLB/Federal Reserve Bank stock	136,206	119,686
Commercial	2,929,558	2,698,023
Commercial real estate	4,458,662	4,333,116
Residential real estate	2,164,283	2,165,290
Consumer credit	1,687,443	1,871,311
Accrued interest receivable	78,493	87,102
Noninterest-bearing demand deposits	3,588,370	3,680,807
Checking, NOW, savings, and money market interest-bearing deposits	7,117,695	7,290,521
Time deposits	1,892,135	1,634,436
Federal funds purchased and interbank borrowings	450,031	335,033
Securities sold under agreements to repurchase	319,831	384,810
FHLB advances	1,554,515	1,609,579
Other borrowings	251,662	248,782
Accrued interest payable	6,582	7,029
Standby letters of credit	445	351
US Government Agencies Debt Securities [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	70,632
US Government Agencies Debt Securities [Member] \| Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	73,776
Mortgage-Backed Securities - Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	131,562	6,903
Mortgage-Backed Securities - Agency [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	126,612	7,056
Mortgage-Backed Securities - Agency [Member] \| Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	131,562	6,903
States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	306,869	677,160
States and Political Subdivisions [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	308,322	720,647
States and Political Subdivisions [Member] \| Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities held to maturity	$ 306,869	$ 677,160

Checking, NOW, savings, and money market interest-bearing deposits.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

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Amount of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

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Including both current and noncurrent portions, Federal Home Loan Bank borrowings initially due beyond one year or beyond the normal operating cycle if longer.

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Amount of Federal Home Loan Bank (FHLB) and Federal Reserve Bank stock.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of interest payable on debt, including, but not limited to, trade payables.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount before allowance of installment and revolving loans issued to individuals, including, but not limited to credit card, automobile, boat, student, durable home appliance loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount before allowance of consumer real estate loans. Includes, but not limited to, construction financing, residential mortgage loans, and home equity lines of credit. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount before allowance of commercial loans issued to businesses to acquire, develop, construct, improve, or refinance land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

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Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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